Method of quenching sheared portion, steel product including sheared portion that is quenched and method of manufacturing steel product including sheared portion that is quenched

ABSTRACT

A method of quenching a sheared portion includes a heating process heating a steel product to a temperature equal to or greater than an austenitizing temperature, a shearing process shearing the steel product which is heated in the heating process to form a sheared portion at the steel product while the steel product is maintained at the temperature equal to or greater than the austenitizing temperature, and a cooling process cooling the steel product at which the sheared portion is formed in the shearing process to a temperature equal to or smaller than a martensitic transformation start point.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 U.S.C. §119 to Japanese Patent Application 2015-010296, filed on Jan. 22, 2015, the entire content of which is incorporated herein by reference.

TECHNICAL FIELD

This disclosure generally relates to a method of quenching a sheared portion, a steel product including the sheared portion that is quenched and a method of manufacturing the steel product including the sheared portion that is quenched.

BACKGROUND DISCUSSION

A steel product that is heated to a temperature equal to or greater than an austenitizing temperature is sheared (i.e., formed) while being retained and held by a die and a stripper disposed within a die device, and is cooled to a temperature equal to or smaller than a martensitic transformation start point so that a sheared portion formed at the steel product is quenched. Such quenching method is called a simultaneous forming and quenching method because forming and quenching are performed and promoted simultaneously.

JP2002-339015A which is hereinafter referred to as Reference 1 discloses the simultaneous forming and quenching method where an alloy steel sheet or a carbon steel sheet is press-formed and quenched at the same time. Reference 1 describes that it is more difficult to quench a sheared portion that is formed by the shearing in the simultaneous forming and quenching than to quench a portion which is not sheared, i.e., a non-sheared portion. In addition, Reference 1 describes that, in a case where a DI value representing an ideal critical diameter of the steel sheet which serves as a workpiece for the simultaneous forming and quenching is equal to or greater than 1.8 inches or equal to or greater than 3.6 inches, the sheared portion is effectively quenched.

The steel product including a high DI value, however, is expensive because of a great number of additive elements.

A need thus exists for a method of quenching a sheared portion, a steel product including the sheared portion that is quenched and a method of manufacturing the steel product including the sheared portion that is quenched which are not susceptible to the drawback mentioned above.

SUMMARY

According to an aspect of this disclosure, a method of quenching a sheared portion includes a heating process heating a steel product to a temperature equal to or greater than an austenitizing temperature, a shearing process shearing the steel product which is heated in the heating process to form a sheared portion at the steel product while the steel product is maintained at the temperature equal to or greater than the austenitizing temperature, and a cooling process cooling the steel product at which the sheared portion is formed in the shearing process to a temperature equal to or smaller than a martensitic transformation start point.

According to another aspect of this disclosure, a steel product includes a sheared portion formed by a shearing, a portion including the sheared portion being quenched, a crystal grain size at the sheared portion being equal to or greater than 4 μm, and a DI value less than 1.8 inches.

According to a further aspect of this disclosure, a method of manufacturing a steel product including a sheared portion which is quenched includes a heating process heating a steel product to a temperature equal to or greater than an austenitizing temperature, the steel product including a DI value less than 1.8 inches, a shearing process shearing the steel product which is heated in the heating process to form a sheared portion at the steel product while the steel product is maintained at the temperature equal to or greater than the austenitizing temperature, and a cooling process cooling the steel product at which the sheared portion is formed in the shearing process to a temperature equal to or smaller than a martensitic transformation start point.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and additional features and characteristics of this disclosure will become more apparent from the following detailed description considered with the reference to the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view schematically illustrating a die device used for a shearing process according to an embodiment disclosed here;

FIG. 2 is a cross-sectional view schematically illustrating the die device at which an S22CB steel sheet is set;

FIG. 3 is a cross-sectional view schematically illustrating the die device where the S22CB steel sheet is retained and held by a die and a stripper;

FIG. 4 is a cross-sectional view schematically illustrating the die device where the S22CB steel sheet is punched by a punch;

FIG. 5 is a cross-sectional view schematically illustrating the die device where the S22CB steel sheet is kept warm onto the die;

FIG. 6 is a cross-sectional view schematically illustrating the die device from which the S22CB steel sheet is taken;

FIG. 7 is a diagram illustrating a temperature change of the S22CB steel sheet in a case where a heating process, a shearing process, a heat reserving process and a cooling process are performed;

FIG. 8 is a plan view of each sample according to a first example, a second example and a comparative example;

FIG. 9 is a cross-sectional view taken along a line IX-IX in FIG. 8;

FIGS. 10A, 10B and 10C are electron microscope pictures of cross-sections of upper sheared portions of the respective samples;

FIGS. 11A, 11B and 11C are electron microscope pictures of cross-sections of lower sheared portions of the respective samples; and

FIGS. 12A, 12B and 12C are electron microscope pictures indicating crystal grains at the lower sheared portions of the respective samples.

DETAILED DESCRIPTION

An embodiment is explained with reference to the attached drawings. In the embodiment, a sheared portion is formed at S22CB steel sheet serving as a kind of boron steel sheet which is a steel product including a low DI value. A method of quenching the sheared portion formed at the S22CB steel sheet and a method of manufacturing the S22CB steel sheet including the sheared portion which is quenched are explained. The S22CB steel sheet is inexpensive and a DI value thereof is 1.5 inches.

Each of the aforementioned quenching method and manufacturing method includes a heating process, a shearing process, a heat reserving process and a cooling process.

In the heating process, the S22CB steel sheet is heated so that the temperature thereof increases to or over an austenitizing temperature, i.e., substantially 96° C., for example. A heating method is not specific. For example, the S22CB steel sheet may be heated by a heating furnace (oven). Alternatively, the S22CB steel sheet may be heated by a radio frequency heating apparatus.

In the shearing process, the S22CB steel sheet is sheared while the S22CB steel sheet is maintained at the temperature equal to or greater than the austenitizing temperature to thereby form and obtain the sheared portion at the S22CB steel sheet. In the present embodiment, a penetration bore is formed at the S22CB steel sheet by punching with a die device. Thus, an inner wall surface of the penetration bore formed in the aforementioned manner corresponds to a sheared surface. In addition, the aforementioned inner wall surface of the penetration bore and a portion immediately below the inner wall surface (i.e., a portion around the inner wall surface) corresponds to the sheared portion. As illustrated in FIG. 1, the die device, i.e., a die device 1, includes an upper die portion 2 and a lower die portion 3.

The upper die portion 2 includes an upper-side plate portion 21, a stripper 22, plural poles 23, plural springs 24 and a punch 25. In the embodiment, the upper-side plate portion 21 is configured by a lamination of three plates corresponding to an upper plate 21 a, an intermediate plate 21 b and a lower plate 21 c. Penetration bores are formed at respective center portions of the intermediate plate 21 b and the lower plate 21 c. The diameter of the penetration bore formed at the intermediate plate 21 b is greater than the diameter of the penetration bore formed at the lower plate 21 c. The intermediate plate 21 b and the lower plate 21 c are laminated each other in a state where the aforementioned penetration bores are coaxially disposed to each other. As a result, a stepped portion is formed at a boundary between a lower opening surface of the penetration bore formed at the intermediate plate 21 b and an upper opening surface of the penetration bore formed at the lower plate 21 c.

The punch 25, which is formed in a stepped column configuration, includes a fixation portion 25 a, an intermediate portion 25 b and a punch portion 25 c. The fixation portion 25 a, the intermediate portion 25 b and the punch portion 25 c are coaxially formed in the mentioned order along an axial direction of the punch 25. The diameter of the fixation portion 25 a is greater than the diameter of the intermediate portion 25 b, and the diameter of the intermediate portion 25 b is greater than the diameter of the punch portion 25 c. The fixation portion 25 a is disposed within the penetration bore formed at the intermediate plate 21 b and the intermediate portion 25 b is disposed within the penetration bore formed at the lower plate 21 c. Then, a lower surface of the fixation portion 25 a is retained at the stepped portion formed at the boundary between the lower opening surface of the penetration bore formed at the intermediate plate 21 b and the upper opening surface of the penetration bore formed at the lower plate 21 c. As a result, the punch 25 is fixed to the upper-side plate portion 21 so that the punch 25 and the upper-side plate portion 21 integrally rotate. The punch portion 25 c protrudes to a lower side from the lower plate 21 c.

The stripper 22 is disposed at a lower side of the upper-side plate portion 21. A punch penetration bore 22 a serving as a penetration bore for the punch portion 25 c is formed at a center portion of the stripper 22. Pole penetration bores 22 b serving as penetration bores for the respective poles 23 are formed in the vicinity of four corners of the stripper 22.

The poles 23 are fixed in the vicinity of four corners of the lower plate 21 c. Each of the poles 23 protrudes in a bar form downwardly from a lower surface of the lower plate 21 c. A lower portion of each of the poles 23 enters the pole penetration bore 22 b formed at the stripper 22 from an upper surface side of the stripper 22 so as to be positioned within the pole penetration bore 22 b. As illustrated in FIG. 1, a diameter of an upper side portion of the pole penetration bore 22 b is smaller than a diameter of a lower side portion of the pole penetration bore 22 b. Thus, a stepped portion is formed at a boundary between the upper side portion including a smaller diameter (i.e., small diameter portion) and the lower side portion including a larger diameter (i.e., large diameter portion). In addition, an enlarged diameter portion extending radially outwardly is formed at a distal end (lower end) of the pole 23. The enlarged diameter portion is retained at the aforementioned stepped portion within the pole penetration bore 22 b so that the stripper 22 is suspended from the upper-side plate portion 21 via the poles 23. At this time, the punch portion 25 c of the punch 25 is inserted to be positioned within the punch penetration bore 22 a formed at the stripper 22.

The springs 24 are wound around the poles 23 respectively. An upper end of each of the springs 24 in FIG. 1 is fixed to (or engaged with) the lower surface of the lower plate 21 c and a lower end of each of the springs 24 in FIG. 1 is fixed to (or engaged with) an upper surface of the stripper 22. The spring 24 constantly generates a stretching force or an extending force in a state being wound around the pole 23. Therefore, the stripper 22 is biased in a direction away from the upper-side plate portion 21 by the springs 24. In this case, the enlarged diameter portion formed at the distal end of the pole 23 engages with the stepped portion formed at the pole penetration bore 22 b to inhibit the stripper 22 from disengaging from the pole 23 by a biasing force of the spring 24.

The lower die portion 3 includes a die 31 and a base plate 32. The die 31 is fixed to an upper surface of the base plate 32. An S22CB steel sheet W (steel product) is placed onto an upper surface of the die 31. A punching penetration bore 31 a serving as a penetration bore for punching is formed at a center portion of the die 31. The punching penetration bore 31 a opens at the upper surface of the die 31. The position of the die 31 relative to the stripper 22 is decided so that the punching penetration bore 31 a is coaxially arranged with the punch penetration bore 22 a formed at the stripper 22.

A die and a stripper provided at a die device are generally made of hot work die steel (for example, SKD61). In the embodiment, however, the die 31 and the stripper 22 are made of zirconia (ZrO₂) ceramics (zirconia-based ceramics). A heat conductivity of zirconia ceramics is equal to or smaller than 5 W/m-K (specifically, 3 W/m-K) which is considerably lower than a heat conductivity of hot work die steel. That is, in the present embodiment, the die 31 and the stripper 22 are made of the material including a lower heat conductivity as compared to the normal die and stripper.

With the die device 1 configured in the aforementioned manner, the penetration bore is formed at the S22CB steel sheet W. As illustrated in FIG. 1, the S22CB steel sheet W is a plate-shaped member including a lower surface WL serving as a first surface and an upper surface WU serving as a second surface provided at an opposite side from the lower surface WL. In a case of forming the penetration bore at the S22CB steel sheet W, first, the S22CB plate sheet W that is heated in the heating process is placed onto a region at the upper surface of the die 31 provided at the die device 1, the region including a portion at which the punching penetration bore 31 a is opened. At this time, the lower surface WL of the S22CB steel sheet W makes contact with the upper surface of the die 31. The S22CB steel sheet W is thus set within the die device 1 as illustrated in FIG. 2.

After the S22CB steel sheet W is set within the die device 1, the upper die portion 2 is lowered towards the lower die portion 3. A lower surface of the stripper 22 thus makes contact with the upper surface WU of the S22CB steel sheet W. The S22CB steel sheet W is then retained and held (i.e., locked) by the die 31 and the stripper 22 as illustrated in FIG. 3.

Afterwards, the upper-side plate portion 21 is further lowered, which causes the punch 25 that moves integrally with the upper-side plate portion 21 to be lowered. As a result, the punch 25 punches through the S22CB steel sheet W to form and obtain the penetration bore at the S22CB steel sheet W, the penetration bore penetrating through the S22CB steel sheet W from the upper surface WU to the lower surface WL as illustrated in FIG. 4.

Accordingly, the S22CB steel sheet W is sheared (i.e., punched) by the punch 25 in a state being retained and held by the die 31 and the stripper 22 provided at the die device 1, i.e., in a state making contact with the die 31 and the stripper 22. At this time, the die 31 and the stripper 22 are made of zirconia ceramics including the low heat conductivity as mentioned above. Thus, the heat of the S22CB steel sheet W is difficult to be transmitted to the die 31 and the stripper 22. The decrease of temperature of the S22CB steel sheet W at the shearing process is small. As a result, the S22CB steel sheet W may be maintained at the temperature equal to or greater than the austenitizing temperature at the shearing. In addition, the shearing (punching) may be performed on the S22CB steel sheet W in the aforementioned state where the S22CB steel sheet W is maintained at the temperature equal to or greater than the austenitizing temperature. That is, in the present embodiment, the S22CB steel sheet W is sheared (punched) while the S22CB steel sheet W which is heated in the heating process is maintained at the temperature equal to or greater than the austenitizing temperature so as to form and obtain the sheared portion at the S22CB steel sheet W.

After the shearing (punching) process is completed, the upper die portion 2 of the die device 1 is raised or lifted up so that the stripper 22 is separated from the upper surface WU of the S22CB steel sheet W. The S22CB steel sheet W placed onto the die 31 is left for a predetermined time period. A sheared surface of the S22CB steel sheet W makes contact with the punch 25 at the shearing of the S22CB steel sheet W. Thus, the sheared surface is deprived of heat by the punch 25. Thus, the temperature of the sheared portion slightly decreases. Nevertheless, because of a small contact area between the punch 25 and the S22Cb steel sheet W, the temperature of the sheared portion is inhibited from extremely decreasing below the austenitizing temperature. On the other hand, a portion on which the shearing is not performed, i.e., a non-sheared portion, of the S22CB steel sheet W is not in contact with the punch 25. The temperature of the non-sheared portion is therefore higher than the sheared portion. After the shearing, therefore, the heat is transmitted (i.e., flows) from the non-sheared portion that is not in contact with the punch 25 to the sheared portion that makes contact with the punch 25 so that the temperature of the sheared portion slightly increases. That is, the heat of the S22CB steel sheet W is reserved or raised by simply leaving the S22CB steel sheet W onto the die 31 after the shearing (i.e., heat reserving process). Because of the aforementioned heat reserving process, the S22CB steel sheet W at which the sheared portion is formed in the shearing process is maintained at the temperature equal to or greater than the austenitizing temperature as illustrated in FIG. 5. The heat reserving process may be a process in which the S22CB steel sheet W is taken from the die device 1 and left in air or in a heat insulating tank, for example.

After the heart reversing process is performed (i.e., the S22CB steel sheet W is left onto the die 31) for the predetermined time period, the cooling process is conducted. First, the S22CB steel sheet W which is left onto the die 31 is taken from the die device 1 as illustrated in FIG. 6. The S22CB steel sheet W taken from the die device 1 is cooled by a predetermined method. For example, the S22CB steel sheet W taken from the die device 1 is immersed in cooling medium such as water, for example, to cool the S22CB steel sheet W. Alternatively, the S22CB steel sheet W taken from the die device 1 is retained and held (locked) by a die for cooling to cool the S22CB steel sheet W. According to the aforementioned cooling process, the temperature of the S22CB steel sheet W decreases to or below a martensitic transformation start point. Consequently, the S22CB steel sheet W is quenched.

As mentioned above, a portion of the S22CB steel sheet W including the sheared portion is quenched through the heating process, the shearing process, the heart reserving process and the cooling process. In addition, the S22CB steel sheet W including the sheared portion that is quenched through the heating process, the shearing process, the heart reserving process and the cooling process is manufactured.

FIG. 7 is a diagram illustrating a temperature change of the sheared portion of the S22CB steel sheet W in a case where the aforementioned heating process, shearing process, heart reserving process and cooling process are continuously performed on the S22CB steel sheet W. In FIG. 7, a solid line indicates the temperature change of the S22CB steel sheet W according to the present embodiment. A dotted line in FIG. 7 indicates the temperature change of the sheared portion of the S22CB steel sheet in a case where the sheared portion of the S22CB steel sheet is simultaneously formed and quenched with a die device including a die and a stripper made of SKD61. In a vertical axis in FIG. 7, a point A3 indicates the austenitizing temperature and a point Ms is the martensitic transformation start point (the martensitic transformation start temperature).

As illustrated in FIG. 7, in the present embodiment, the sheared portion of the S22CB steel sheet W is maintained at the temperature equal to or greater than the austenitizing temperature (723° C.) during the shearing process and the heat reserving process. In the heat reserving process, the heat flows or is transmitted from the non-sheared portion to the sheared portion as mentioned above, which gently increases the temperature of the sheared portion. On the other hand, in a case where the S22CB steel sheet is retained and held by the die and the stripper made of SKD61 when the known simultaneous forming and quenching method is performed, the temperature of the sheared portion of the S22CB steel sheet rapidly decreases.

Then, in the shearing process, the temperature of the sheared portion of the S22CB steel sheet decreases below the austenitizing temperature (723° C.).

In a case where a steel product is sheared, dynamic recrystallization occurs at a portion that is sheared (sheared portion). As a result, fine grains (crystal grains) are generated at the sheared portion. The generated fine grains grow under a high temperature condition. Specifically, in a case where the temperature of the steel product (temperature of the sheared portion) is equal to or greater than the austenitizing temperature, the fine grains generated at the sheared portion of the steel product grow.

In addition, the grain size of each of the fine grains generated at the sheared portion, i.e., crystal grain size, influences a quenching performance at the sheared portion. Specifically, the smaller the fine grain size is, the severer a quenching condition (cooling condition) is, i.e., the greater the crystal grain size is, the milder the quenching condition is. The quenching condition (cooling condition) becomes milder with an increase of the crystal grain size, which results in easy quenching.

In light of the foregoing, according to the embodiment, the steel product (i.e., S22CB steel sheet W) is maintained at the temperature equal to or greater than the austenitizing temperature at the shearing process and the following heat reserving process. Thus, the fine grains generated at the sheared portion sufficiently grow during and after the shearing process. The crystal grain size at the sheared portion at the start of the cooling process is sufficiently large. Accordingly, in a case where the S22CB steel sheet W is cooled to or below the martensitic transformation start point, the sheared portion may be securely quenched. In addition, because of the aforementioned heating process, shearing process, heat reserving process and cooling process, the portion including the sheared portion formed at the steel product such as boron steel, for example, including the DI value less than 1.8 inches may be adequately quenched.

A sample S1 according to a first example is prepared as below. First, the S22CB steel sheet in a flat plate form is heated to 960° C. (heating process). Next, the S22CB steel sheet is set within the die device so that the lower surface of the S22CB steel sheet that is heated makes contact with the upper surface of the die made of zirconia ceramics. The stripper made of zirconia ceramics disposed within the die device is then lowered so that the lower surface of the stripper makes contact with the upper surface of the S22CB steel sheet. As a result, the S22CB steel sheet is retained and held (locked) by the die and the stripper made of zirconia ceramics. While the S22CB steel sheet is being retained and held by the die and the stripper, the punch made of SKD61 is lowered to punch the S22CB steel sheet from the upper surface to the lower surface thereof. The penetration bore is thus formed at the S22CB steel sheet (shearing process). The stripper is thereafter lifted up so as to leave the S22CB steel sheet onto the die to keep and maintain the heat of the S22CB steel sheet (heat reserving process). After an elapse of the predetermined time period, the S22CB steel sheet is taken from the die device and is cooled in water (cooling process). According to the aforementioned processes, the sample S1 serving as the S22CB steel sheet including the sheared portion is manufactured.

A second sample S2 according to a second example is prepared as below. First, the S22CB steel sheet in a flat plate form is heated to 960° C. (heating process). Next, the S22CB steel sheet is set within the die device so that the lower surface of the S22CB steel sheet that is heated makes contact with the upper surface of the die made of zirconia ceramics. The stripper made of zirconia ceramics provided within the die device is then lowered so that the lower surface of the stripper makes contact with the upper surface of the S22CB steel sheet. As a result, the S22CB steel sheet is retained and held (locked) by the die and the stripper made of zirconia ceramics. While the S22CB steel sheet is being retained and held by the die and the stripper, the punch made of zirconia ceramics is lowered to punch the S22CB steel sheet from the upper surface to the lower surface thereof. The penetration bore is thus formed at the S22CB steel sheet (shearing process). The stripper is thereafter lifted up so as to leave the S22CB steel sheet onto the die to keep and maintain the heat of the S22CB steel sheet (heat reserving process). After the elapse of the predetermined time period, the S22CB steel sheet is taken from the die device and is cooled in water (cooling process). According to the aforementioned processes, the sample S2 serving as the S22CB steel sheet including the sheared portion is manufactured.

A sample 3 according to a comparative example is prepared as blow. First, the S22CB steel sheet in a flat plate form is heated to 960° C. (heating process). Next, the S22CB steel sheet is set within the die device so that the lower surface of the S22CB steel sheet that is heated makes contact with the upper surface of the die made of SKD61. The stripper made of SDK61 disposed within the die device is then lowered so that the lower surface of the stripper makes contact with the upper surface of the S22CB steel sheet. As a result, the S22CB steel sheet is retained and held (locked) by the die and the stripper made of SKD61. While the S22CB steel sheet is being retained and held by the die and the stripper, the punch made of SKD61 is lowered to punch the S22CB steel sheet from the upper surface to the lower surface thereof. The penetration bore is thus formed at the S22CB steel sheet (shearing process). The stripper is thereafter lifted up so as to leave the S22CB steel sheet onto the die to keep and maintain the heat of the S22CB steel sheet (heat reserving process). After the elapse of the predetermined time period, the S22CB steel sheet is taken from the die device and is cooled in water (cooling process). According to the aforementioned processes, the sample S3 serving as the S22CB steel sheet including the sheared portion is manufactured.

FIG. 8 is a plan view of each of the aforementioned samples S1, S2 and S3. As illustrated in FIG. 8, a penetration bore H which penetrates through each of the samples S1, S2 and S3 (S22CB steel sheet) from the upper surface to the lower surface is formed. The sample S1, S2, S3 is cut along a line IX-IX in FIG. 8 so that the line IX-IX passes through an edge of the penetration bore H as illustrated in FIG. 9. As illustrated in FIG. 9, a portion in the vicinity of the penetration bore H is plastically deformed by the shearing. Such the portion that is plastically deformed, i.e., an inner wall surface of the penetration bore H and a portion around the inner wall surface (portion immediately below the inner wall surface), corresponds to the sheared portion.

In the sheared portion, a portion in the vicinity of the upper surface of each of the samples S1, S2 and S3 (i.e., surface that makes contact with the stripper when each sample is locked within the die device) corresponding to a portion within a region B in FIG. 9, is deformed to be pulled into the penetration bore H. Such the portion is hereinafter referred to as an upper sheared portion. On the other hand, a portion in the vicinity of the lower surface of each of the samples S1, S2 and S3 (i.e., surface that makes contact with the die when each sample is locked within the die device) corresponding to a portion within a region C in FIG. 9, is deformed to protrude from the penetration bore H. Such the portion is hereinafter referred to as a lower sheared portion.

The upper sheared portion and the lower sheared portion shown in the cross-section along the line IX-IX in FIG. 9 were observed by an electron microscope. FIGS. 10A, 10B and 10C are electron microscope pictures of the cross-sections of the upper sheared portions of the samples S1, S2 and S3 respectively. FIGS. 11A, 11B and 11C are electron microscope pictures of the cross-sections of the lower sheared portions of the respective samples S1, S2 and S3 respectively.

As illustrated in FIGS. 10A, 10B and 10C, the fine grains resulting from the dynamic recrystallization are not greatly generated at the upper sheared portions of the samples S1, S2 and S3. This is because the plastic deformation amount at the sheared portion in the vicinity of the upper surface at the time of punching from the upper surface to the lower surface of each of the samples S1, S2 and S3 is small, i.e., because of a small (low) deformation. Because the fine grains resulting from the dynamic recrystallization are not greatly generated at the portion with the small deformation, the sufficient quenching may be obtained even though the temperature of the S22CB steel sheet is equal to or smaller than the austenitizing temperature at the shearing. Thus, hardness of the upper sheared portion of each of the samples S1, S2 and S3 according to the first example, second example and comparative example is high. Vickers hardness (HV) of the upper sheared portion of the sample S1 is HV500. Vickers hardness (HV) of the upper sheared portion of the sample S2 is HV520. Vickers hardness (HV) of the upper sheared portion of the sample S3 is HV380. Any values of Vickers hardness according to the samples S1, S2 and S3 are equal to or greater than HV350.

On the other hand, as indicated by FIGS. 11A, 11B and 11C, the fine grains resulting from the dynamic recrystallization are generated at the lower sheared portions of the samples S1, S2 and S3. This is because the plastic deformation amount at the sheared portion in the vicinity of the lower surface at the time of punching from the upper surface to the lower surface of each of the samples S1, S2 and S3 is larger, i.e., because of a large (high) deformation. The grain size of each of the generated fine grains (crystal grain size) of each of the samples S1 and S2 is large while the grain size of each of the generated fine grains (crystal grain size) of the sample S3 is small. Accordingly, it is understandable that, in the samples S1 and S2 according to the first and second examples, the fine grains generated at the lower sheared portion sufficiently grow because the S22CB steel sheet is maintained at the temperature equal to or greater than the austenitizing temperature at the shearing process and the heat reserving process. Vickers hardness at the lower sheared portion of the sample S1 is HV 500. Vickers hardness at the lower sheared portion of the sample S2 is HV 520. On the other hand, Vickers hardness at the lower sheared portion of the sample S3 is HV 260. By the aforementioned results, it is understandable that the lower sheared portions of the samples S1 and S2 are sufficiently quenched while the lower sheared portion of the sample S3 is not sufficiently quenched.

FIGS. 12A, 12B and 12C are enlarged views of the lower sheared portions of the samples S1, S2 and S3 respectively and are electron microscope pictures particularly indicating the crystal grains at the respective lower sheared portions. As seen from FIGS. 12A, 12B and 12C, the crystal grain size of the lower sheared portion of each of the samples S1 and S2 is greater than the crystal grain size of the lower sheared portion of the sample S3. Specifically, the crystal grain size at the lower sheared portion of the sample S1 is 4 μm to 8 μm. The crystal grain size at the lower sheared portion of the sample S2 is 6 μm to 8 μm. The crystal grain size at the lower sheared portion of the sample S3 is 1 μm to 2 μm. As a result, it is understandable that the sheared portion is sufficiently quenched in a case where the crystal grain size of the sheared portion is equal to or greater than In addition, the S22CB steel sheet is maintained at the temperature equal to or greater than the austenitizing temperature until the crystal grain size of each of the fine grains generated at the lower sheared portion reaches or exceeds 4 μm in the heat reserving process so as to sufficiently quench the sheared portion.

In the first and second examples, the die and the stripper are made of zirconia ceramics. The usage of the die and the stripper made of zirconia ceramics may restrain the decrease of temperature of the S22CB steel sheet at the shearing. Thus, the S22CB steel sheet may be maintained at the temperature equal to or greater than the austenitizing temperature at the shearing. In addition, the heat conductivity of each of the die and the stripper employed in the first and second examples is 3 W/m-K. The decrease of temperature of the S22CB steel sheet at the shearing may be restrained by the die and the stripper which are made of the material of which the heat conductivity is equal to or smaller than 5 W/m-K. Consequently, the S22CB steel sheet may be maintained at the temperature equal to or greater than the austenitizing temperature at the shearing.

The embodiment is not limited to the above. For example, in the aforementioned embodiment, the S22CB steel sheet serving as a kind of boron steel is used as the steel product. Alternatively, other steel sheet may be employed as the steel product. In addition, in the aforementioned embodiment, the shearing process is achieved in a manner that the penetration bore is formed by punching. Alternatively, any working process may be employed as long as a portion that is sheared is formed. For example, in a case where the steel product is sheared by forging, the method of quenching and the method of manufacturing in the disclosure may be employed.

In addition, in the aforementioned embodiment, the example where the steel sheet including the flat shape is sheared is explained, however, the steel product including any form may be acceptable. Further, the example where the die and the stripper disposed within the die device are made of ceramics is explained in the above. Alternatively, the die and the stripper may be made of other materials than ceramics as long as the heat conductivity of such materials is lower than metal, specifically, lower than SKD61 (hot work die steel). In addition, such materials may be metal, for example, SKD61, as long as the die and the tripper are heated to the temperature equal to or greater than the austenitizing temperature of the steel product.

Further, in the aforementioned embodiment, the example where the steel product is kept warm (heated) by leaving the steel product onto the die after the shearing is explained. Alternatively, the steel product may be heated by heating means so that the steel product after the shearing is maintained at the temperature equal to or greater than the austenitizing temperature. The example where the shearing is conducted while the S22CB steel sheet is retained and held by the die and the stripper is also explained in the above. Alternatively, for example, the steel product may be sheared while the surroundings of the portion on which the shearing is performed is not retained or held. In this case, the vicinity of the sheared portion may be inhibited from making contact with the other members. Thus, the shearing may be conducted in a state where the steel product is maintained at the temperature equal to or greater than the austenitizing temperature. As mentioned above, the embodiment may be appropriately changed or modified without departing the scope thereof.

According to the known simultaneous forming and quenching method, in a case where a steel sheet (steel product) that is heated to or over the austenitizing temperature is sheared within a die device, the steel sheet is retained and held by a die and a stripper disposed within the die device. The die and the stripper used for the simultaneous forming and quenching are made of hot work die steel (for example, SKD61). In a case where the steel sheet is retained and held by the die and the stripper made of hot work die steel, heat of the steel sheet is taken away by the die and the stripper. Accordingly, the temperature of the steel sheet (specifically, the temperature of the sheared portion) decreases below the austenitizing temperature at the shearing.

In addition, the fine grains (crystal grains) are generated due to the dynamic recrystallization at the sheared portion (specifically, heavy deformed portion) which is formed at the steel sheet by the shearing. The fine grains generated at the sheared portion grow in a case where the temperature of the steel sheet is equal to or greater than the austenitizing temperature. According to the known simultaneous forming and quenching method, the temperature of the steel sheet decreases below the austenitizing temperature. Thus, the fine grains generated at the sheared portion after the shearing are inhibited from growing. As a result, the crystal grain size at the sheared portion is small. With the small crystal grain size at the sheared portion, the condition for quenching (for example, cooling condition) is severe and therefore quenching failure (lack of quenching) may occur at the sheared portion. At this time, a steel sheet (steel product) with a high DI value so as to have an improved quenching performance may be used for enhancing the quenching performance of the sheared portion, however, the steel sheet including the high DI value is expensive.

On the other hand, according to the embodiment, the shearing is performed while the steel sheet (steel product) is maintained at the temperature equal to or greater than the austenitizing temperature. That is, the shearing is completed within a temperature range (γ region) of the steel sheet equal to or greater than the austenitizing temperature. Therefore, the fine grains generated at the sheared portion grow after the shearing. The crystal grain size at the sheared portion is thus large. In a case where the crystal grain size at the sheared portion is large, the condition for quenching is milder than a case where the crystal grain size is small. As a result, the quenching of the sheared portion may be easily performed. That is, in the present embodiment, the steel sheet is sheared within the temperature range equal to or greater than the austenitizing temperature so as to grow the fine grains generated at the sheared portion, thereby improving the quenching performance at the sheared portion. Consequently, even with the steel sheet including the low DI value, i.e., with the steel sheet including the DI value less than 1.8 inches, for example, the sheared portion formed at such the steel sheet may be successfully quenched.

In the embodiment, the austenitizing temperature corresponds to a temperature at which austenite transformation is started. In addition, the sheared portion corresponds to a portion to which a shearing force is applied by the shearing. Thus, the sheared surface formed by the shearing process and the portion immediately below the sheared surface correspond to the sheared portion.

In the embodiment, the method of quenching the sheared portion includes the heat reserving process maintaining the S22CB steel sheet (steel product) at which the sheared portion is formed in the shearing process at the temperature equal to or greater than the austenitizing temperature. The cooling process is performed after the heat reserving process is performed.

Accordingly, because the steel sheet is maintained at the temperature equal to or greater than the austenitizing temperature in the heat reserving process, the fine grains generated at the sheared portion in the shearing process may sufficiently grow. That is, the crystal grain size at the sheared portion may be enlarged. As a result, when the steel sheet is thereafter cooled to the temperature equal to or smaller than the martensitic transformation start point in the cooling process, the sheared portion formed at the steel sheet may be securely quenched.

In the embodiment, in the heat reserving process, the steel sheet is maintained at the temperature equal to or greater than the austenitizing temperature until the crystal grain size at the sheared portion becomes equal to or greater than

Accordingly, the sheared portion formed at the steel sheet may be further securely quenched.

In the embodiment, the steel sheet is a plate-shaped member including the lower surface (first surface) WL and the upper surface (second surface) WU which is disposed at the opposite side from the lower surface WL. The shearing process is performed with the die device 1 including the die 31 onto which the steel sheet is placed and which makes contact with the lower surface WL of the steel sheet, the stripper 22 which makes contact with the upper surface WU of the steel sheet placed onto the die 31, and the punch 25 for shearing the steel product. At least the die 31 and the stripper 22 are made of a material including a lower heat conductivity than a metal.

The die 31 and the stripper 22 may be desirably made of a material including a low heat conductivity than a conductivity of SKD61 serving as hot work die steel. The die 31 and the stripper 22 may be further desirably made of a material including a heat conductivity equal to or smaller than 5 W/m-K.

Accordingly, components which are in contact with a workpiece (steel sheet) for a long time period, such as the die 31 and the stripper 22, for example, among components of the die device 1 employed for the shearing process, are formed of a low heat conductive material to thereby decrease an amount of heat taken from the steel sheet by the die 31 and the stripper 22 during the shearing. Thus, the state where the temperature of the steel sheet is equal to or greater than the austenitizing temperature is maintained even in a case where the steel sheet is retained and held by the die 31 and the stripper 22, and the steel sheet may be sheared by punching while the aforementioned state is maintained.

In the embodiment, the die 31 and the stripper 22 are made of ceramics.

In addition, the ceramics are zirconia ceramics.

The heat conductivity of ceramics is generally low. Specifically, the heat conductivity of zirconia ceramics is considerably lower than the heat conductivity of SKD61. Thus, the die 31 and the stripper 22 made of such ceramics material may further decrease the amount of heat taken from the steel sheet by the die 31 and the stripper 22 during the shearing. The steel sheet may be securely maintained at the temperature equal to or greater than the austenitizing temperature at the shearing.

In the embodiment, the steel sheet includes the sheared portion formed by the shearing, the portion including the sheared portion being quenched, the crystal grain size at the sheared portion being equal to or greater than 4 μm, and the DI value less than 1.8 inches.

In this case, the steel sheet may be desirably boron steel sheet (boron steel). Accordingly, because the crystal grain size at the sheared portion formed at the steel sheet including the low DI value such as boron steel sheet, for example, is equal to or greater than 4 μm, the quenching performance at the sheared portion may be improved. As a result, the steel sheet including the sheared portion which includes the low DI value and which is quenched successfully may be provided.

In the embodiment, the method of manufacturing the steel sheet including a sheared portion which is quenched includes a heating process heating a steel product to a temperature equal to or greater than an austenitizing temperature, the steel product including a DI value less than 1.8 inches, a shearing process shearing the steel product which is heated in the heating process to form a sheared portion at the steel product while the steel product is maintained at the temperature equal to or greater than the austenitizing temperature, and a cooling process cooling the steel product at which the sheared portion is formed in the shearing process to a temperature equal to or smaller than a martensitic transformation start point.

Accordingly, the steel sheet including the sheared portion which includes the low DI value and which is quenched successfully may be manufactured.

The principles, preferred embodiment and mode of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby. 

1. A method of quenching a sheared portion, comprising: a heating process heating a steel product to a temperature equal to or greater than an austenitizing temperature; a shearing process shearing the steel product which is heated in the heating process to form a sheared portion at the steel product while the steel product is maintained at the temperature equal to or greater than the austenitizing temperature; and a cooling process cooling the steel product at which the sheared portion is formed in the shearing process to a temperature equal to or smaller than a martensitic transformation start point.
 2. The method according to claim 1, further comprising a heat reserving process maintaining the steel product at the temperature equal to or greater than the austenitizing temperature after the sheared portion is formed at the steel product in the shearing process, wherein the cooling process is performed after the heat reserving process is performed.
 3. The method according to claim 2, wherein in the heat reserving process, the steel product is maintained at the temperature equal to or greater than the austenitizing temperature until a crystal grain size at the sheared portion becomes equal to or greater than 4 μm.
 4. The method according to claim 1, wherein the steel product is a plate-shaped member including a first surface and a second surface which is disposed at an opposite side from the first surface, the shearing process is performed with a die device including a die onto which the steel product is placed and which makes contact with the first surface of the steel product, a stripper which makes contact with the second surface of the steel product placed onto the die, and a punch for shearing the steel product, at least the die and the stripper are made of a material including a lower heat conductivity than a metal.
 5. The method according to claim 4, wherein the die and the stripper are made of ceramics.
 6. The method according to claim 5, wherein the ceramics are zirconia ceramics.
 7. A steel product comprising: a sheared portion formed by a shearing; a portion including the sheared portion and being quenched; a crystal grain size at the sheared portion being equal to or greater than 4 μm; and a DI value less than 1.8 inches.
 8. A method of manufacturing a steel product including a sheared portion which is quenched, comprising: a heating process heating a steel product to a temperature equal to or greater than an austenitizing temperature, the steel product including a DI value less than 1.8 inches; a shearing process shearing the steel product which is heated in the heating process to form a sheared portion at the steel product while the steel product is maintained at the temperature equal to or greater than the austenitizing temperature; and a cooling process cooling the steel product at which the sheared portion is formed in the shearing process to a temperature equal to or smaller than a martensitic transformation start point. 